THREONINE (abbreviated as THR or T) encoded by the codons ACU, ACC,
ACA, and ACG is an α-amino acid that is used in the biosynthesis of
proteins . It contains an α-amino group (which is in the protonated
3 form under biological conditions), an α-carboxylic acid group
(which is in the deprotonated −COO− form under biological
conditions), and an alcohol containing side chain, classifying it as a
polar, uncharged (at physiological pH) amino acid. It is essential in
humans, meaning the body cannot synthesize it, and must be ingested in
* 1 Stereoisomerism
* 2 History
The threonine residue is susceptible to numerous posttranslational modifications . The hydroxyl side-chain can undergo O-linked glycosylation . In addition, threonine residues undergo phosphorylation through the action of a threonine kinase . In its phosphorylated form, it can be referred to as phosphothreonine .
It is a precursor of glycine , and can be used as a prodrug to reliably elevate brain glycine levels.
As an essential amino acid, threonine is not synthesized in humans, hence we must ingest threonine in the form of threonine-containing proteins. In plants and microorganisms, threonine is synthesized from aspartic acid via α-aspartyl-semialdehyde and homoserine . Homoserine undergoes O-phosphorylation; this phosphate ester undergoes hydrolysis concomitant with relocation of the OH group. Enzymes involved in a typical biosynthesis of threonine include:
* aspartokinase * β-aspartate semialdehyde dehydrogenase * homoserine dehydrogenase * homoserine kinase * threonine synthase .
* It is converted to pyruvate via threonine dehydrogenase . An intermediate in this pathway can undergo thiolysis with CoA to produce acetyl-CoA and glycine . * In humans, it is converted to α-ketobutyrate . (The gene for threonine dehydrogenase is a pseudogene in humans). This is the primary pathway for threonine degradation. The mechanism of the first step is analogous to that catalyzed by serine dehydratase, and the serine and threonine dehydratase may actually be the same enzyme.
The Food and Nutrition Board (FNB) of the U.S. Institute of Medicine set Recommended Dietary Allowances (RDAs) for essential amino acids in 2002. For threonine, for adults 19 years and older, 20 mg/kg body weight/day.
* ^ Dawson, R.M.C., et al., Data for Biochemical Research, Oxford,
Clarendon Press, 1959.
* ^ Raïs, Badr; Chassagnole, Christophe; Lettelier, Thierry; Fell,
David; Mazat, Jean-Pierre (2001). "